U.S. Pat. No. 5,540,299 (hereinafter referred to as “U.S. Pat. No. 5,540,299A”) discloses a method of driving a vehicle having an engine 2 for driving front wheels 1FL, 1FR (primarily driven wheels) and motors ML, MR for driving rear wheels 1RL, 1RR (secondarily driven wheels) (see, Abstract, FIG. 1, and Claim 1). According to U.S. Pat. No. 5,540,299A, when a lateral G (lateral acceleration) is of a predetermined value or greater (W23: YES in FIG. 15), execution of normal driving is inhibited, in view of the fact that it is preferred to leave control over the posture of the vehicle body under the manual operation of the driver (see, W31 in FIG. 15, and column 22, lines 22 through 33). The term “normal driving” implies an assisting drive, and is defined as an antonym of the term “reverse driving”, which implies braking (see, column 8, lines 55 through 59). When the answer to W23 in FIG. 15 is YES, it also is made possible to forcibly execute normal driving in an independent mode, from the viewpoint that it is preferred that stability of the vehicle is improved by operating the vehicle in a four-wheel drive mode (see, column 22, lines 33 through 40).
A four-wheel drive vehicle has also been proposed in which the front and rear wheels thereof can be driven independently of each other (see, U.S. Patent Application Publication No. 2012/0015772, hereinafter referred to as “US2012/0015772A1”). According to US2012/0015772A1, a drive unit 6, which comprises an internal combustion engine 4 and an electric motor 5 arranged in series, drives the front wheels Wf, whereas electric motors 2A, 2B drive the rear wheels Wr (see, FIG. 1, and paragraphs [0084], [0085]).